Photovoltaic solar cells for directly converting radiant energy from the sun into electrical energy are well known. The manufacture of photovoltaic solar cells involves provision of flat semiconductor substrates having a shallow p-n junction adjacent one surface thereof (commonly called the "front surface"). Such substrates are often referred to as "solar cell wafers". A typical solar cell wafer may take the form of a rectangular EFG-grown polycrystalline silicon substrate of p-type conductivity having a thickness in the range of 0.010 to 0.018 inches and a p-n junction located about 0.3-0.5 microns from its front surface. Circular or square single crystal silicon substrates and rectangular cast polycrystalline silicon substrates also are commonly used to make solar cells. The solar cell wafers are converted to finished solar cells by providing them with electrical contacts (sometimes referred to as "electrodes") on both the front and rear sides of the semiconductor substrate, so as to permit recovery of an electrical current from the cells when they are exposed to solar radiation. These contacts are typically made of aluminum, silver, nickel or another metal or metal alloy. A common preferred arrangement is to provide silicon solar cells with rear contacts made of aluminum and front contacts made of silver. The contact on the front surface of the cell is generally in the form of a grid, comprising an array of narrow fingers and at least one elongate bus (commonly called a "bus bar") that intersects the fingers. The width and number of the fingers and bus bars are selected so as to maximize the output current.
Further, to improve the conversion efficiency of the cell, it is accepted practice to form on the front surfaces of the solar cells an electrically non-conducting anti-reflection ("AR") coating that is transparent to solar radiation. In the case of silicon solar cells, the AR coating is often made of silicon nitride or an oxide of silicon or titanium. Typically the AR coating is about 800 Angstroms thick. The AR coating overlies and is bonded to those areas of the front surface of the cell that are not covered by the front contact, except that at least a portion of the front contact (usually a bus bar) is not covered with the AR coating, so as to permit making a soldered connection to that contact.
Photovoltaic solar cells (e.g., silicon solar cells) are relatively small in size, typically measuring 2-4 inches on a side in the case of cells made from rectangular EFG-grown substrates, with the result that their power output also is small. Hence, industry practice is to combine a plurality of cells so as to form a physically integrated module with a correspondingly greater power output. Several solar modules may be connected together to form a larger array with a correspondingly greater power output.
The usual practice is to form a module from two or more "strings" of solar cells, with each string consisting of a plurality of cells arranged in a straight row and electrically connected in series, and the several strings being arranged physically in parallel with one another so as to form an array of cells arranged in parallel rows and columns with spaces between adjacent cells. The several strings are electrically connected to one another in a selected parallel and/or series electrical circuit arrangement, according to voltage and current requirements. A common practice is to use solder coated copper wire, preferably in the form of a flat ribbon, to interconnect a plurality of cells in a string, with each ribbon being soldered to the front or back contact of a particular cell, e.g., by means of a suitable solder paste.
For various reasons including convenience of manufacture and assembly, cost control, and protection of the individual cells and their interconnections, it has been common practice for such modules to have laminated structures. These laminated structures consist of front and back protective sheets, with at least the front sheet serving as a cover and being made of clear glass or a suitable plastic material that is transparent to solar radiation, and the back sheet serving as a support for the cells and being made of the same or a different material as the front sheet. Disposed between the front and back sheets so as to form a sandwich arrangement are the solar cells and a light-transparent polymer material that encapsulates the solar cells and is also bonded to the front and back sheets so as to physically seal off the cells. The laminated sandwich-style structure is designed to mechanically support the cells and also to protect the cells against damage due to environmental factors such as wind, snow, rain, ice, and solar radiation. The laminated structure typically is fitted into a metal frame which provides mechanical strength for the module, and facilitates combining it with other modules so as to form a larger array or solar panel that can be mounted to a support that is arranged to hold the array of cells at the proper angle to maximize reception of solar radiation.
The art of making solar cells and combining them to make laminated modules is exemplified by the following U.S. Pat. Nos.: 4,751,191 (R. C. Gonsiorawski et al.); 5,074,920 (R. C. Gonsiorawski et al.), 5,118,362 (D. A. St. Angelo et al.); 5,178,685 (J. T. Borenstein et al.); 5,320,684 (J. Amick et al); and 5,478,402 (J. I. Hanoka). The teachings of those patents are incorporated herein by reference thereto.
Unfortunately, when a plurality of cells are arrayed in a module, the total active surface area of the array (i.e., the active area of the front faces of the solar cells) is less than the total area exposed to radiation via the transparent front protective sheet. For the most part this is due to the fact that adjacent cells do not touch each other and also the cells at the periphery of the array may not extend fully to the outer edges of the front protective sheet. Consequently less than all of the solar radiation which is received by the module impinges on active solar cell areas, with the remainder of the received solar radiation impinging on any inactive areas that lie between the cells or border the entire array of cells.
As noted in U.S. Pat. No. 4,235,643, issued Nov. 25, 1980 to James A. Amick for "Solar Cell Module", a number of techniques have been proposed for increasing the efficiency and effectiveness of solar cell modules by concentrating incident solar radiation onto active cell areas. For example, U.S. Pat. No. 2,904,612 describes a reflector-type device in which the land areas between the circular solar calls consist essentially of inverted intersecting frustums of cones circumscribing the cells. Another technique employed to enhance solar cell module output is the use of lenses. Thus U.S. Pat. No. 3,018,313 describes a solar cell module which has an array of lenses covering the module so as to concentrate the light impinging on the cover of the solar cell array to converge downwardly toward the active solar cell area. In U.S. Pat. No. 4,053,327, yet another light focusing arrangement is described wherein the cover of a solar cell module comprises a plurality of converging lenses arranged so as to direct the light incident on the module so that it does not fall on the grid lines of the front electrode of the solar cells in the array.
The Amick patent discloses an improvement over such prior efforts which comprises providing between adjacent cells an optical medium having a plurality of light-reflective facets that are angularly disposed so as to define a plurality of grooves having a depth in the range of 0.001" to 0.025", with the angle at the vertex formed by two mutually converging facets being between 110.degree. and 130.degree., preferably about 120.degree., with the result that light impinging on those facets will be reflected back into the transparent front cover member at an angle .phi. greater than the critical angle, and then reflected again internally from the front surface of the cover member so as to impinge on the solar cells. The term "critical angle" refers to the largest value which the angle of incidence may have for a ray of light passing from a more dense optical medium to a less dense optical medium. If the angle of incidence .phi. exceeds the critical angle, the ray of light will not enter the less dense medium (e.g, air) but will be totally internally reflected back into the denser medium (e.g., the transparent cover sheet).
Amick U.S. Pat. No. 4,235,643 suggests (in column 4) that the faceted region is substantially coplanar with the solar cells and preferably the vertical height of a facet will be equal to the thickness of the solar cell. In column 5 of the Amick patent it is stated that the grooves are machined or molded in the optical medium.
Further information about the Amick invention is provided by the technical paper published by James A. Amick and William T. Kurth, "V-Groove Faceted Reflector For Enhanced Module Output", pp. 1376-1381, Record of IEEE Photovoltaic Specialists Conference--1981. In said article, the authors disclose that the faceted reflector was made of acrylic plastic and had a thin aluminum reflecting layer, with the repeat spacing (peak-to-peak spacing) of the facets being 0.070 inches.
However, the Amick reflector invention was not a commercial success. A primary limitation of the Amick invention was the inability to provide a satisfactory reflector medium at an acceptable cost.
Consequently, notwithstanding the advantages made in the recent years in increasing the energy conversion efficiency of solar cells, there still remains a very definite need for improving the ability of a solar cell module to capture and use available light energy and, more importantly, do so using a reflector medium that is relatively inexpensive to manufacture and is easy to use.